1. Field of the Invention
The present invention relates to a transport roller and a transport mechanism that are used in an image forming apparatus.
2. Description of the Related Art
Ordinarily, in electrophotographic image forming apparatuses such as copy machines, printers, and facsimile machines, a curling phenomenon ordinarily occurs in paper that has passed through a fixing portion that fixes developer on the paper.
This phenomenon is conceivable caused by a difference in temperature between a hot roller and a pressure roller in the fixing portion (amount of moisture evaporation at the paper surface). This temperature difference is produced for the following reasons. Generally, in order for the hot roller of the fixing portion to melt non-fixed developer on the paper to fix the developer to the paper, the hot roller has a heat source inside or near the outer circumferential face of the roller. On the other hand, the pressure roller is configured to receive heat from a contact portion that makes contact with the hot roller. Thus, the surface temperature of the hot roller is high (for example, 160 to 200° C.), while the surface temperature of the pressure roller is lower (for example, 110 to 150° C.), so that a temperature difference exists between the hot roller and the pressure roller.
More specifically, when paper having a constant moisture content receives a temperature history in the fixing portion after passing through a transfer process, a large amount of the moisture of the paper surface on the hot roller side evaporates, while a small amount of the moisture of the paper surface (paper back face) on the pressure roller side evaporates, so the amount of expansion or contraction of cellulose of the like constituting the paper is different for the front and back faces of the paper, and as a result, paper that has passed through the fixing portion is in a curled state.
Thus, when discharging paper curled in this manner onto a discharge tray using a discharge mechanism, there is a reduction in the stackability of the paper, and furthermore the paper is provided to a user in that curled state.
So, as conventional technology, discharge apparatuses have been disclosed that properly discharge paper with curling corrected (for example, see JP H11-65192A (hereinafter, referred to as “Patent Document 1”)). In Patent Document 1, in a transport path that transports paper on which unfixed developer has been fixed by a fixing portion, a plurality of drive rollers, and idler rollers that form pairs with the drive rollers, are disposed in the transport direction, and by passing the paper through a nip portion between each of these drive rollers and idler rollers, paper that curled in the transport process is handled to correct its curling, and then that paper is discharged to a discharge tray.
According to the technology described in above Patent Document 1, the idler rollers that form pairs with the drive rollers press against the drive rollers with biasing force provided by respectively independent pressing means, thus correcting curling of stiff, thick paper.
However, with this technology, correction of curling of paper with a different thickness than thick paper such as standard paper or thin paper is performed with the same sort of pressing force. Thus, when using the technology described in above Patent Document 1, creases, tears, or other damage will occur in paper that is less stiff than thick paper.